In printing devices, such as computer printers and plain-paper fax machines, a sheet of media (e.g. paper, transparencies) is transported from an input tray to a printing engine where the sheet receives graphic or text markings. The sheet is then transported into an output tray.
FIG. 1 illustrates a known type of sheet feed mechanism 5. The input tray 10 accommodates a stack of sheet media 12, and a pick mechanism comprising a friction roller 14 is employed to pick individual sheets from the stack and advance the sheets along a media path 16. To effectively transport a media sheet, it is desirable that the sheet does not slide relative to the friction roller 14 surface. It is also desirable that individual sheets be transported in series rather than being one of multiple overlapping sheets. To avoid sliding and multiple-sheet picking, the pick mechanism is configured so that the friction roller applies a normal force N to the media.
The normal force of the friction roller on the media may be achieved by spring-loading the friction roller, or by allowing the weight of the friction roller to act directly on the stack. With the normal force applied, a drive motor rotates the friction roller in an anti-clockwise direction to pick and advance the top-most sheet of the stack. A gear mechanism may be employed to couple the drive motor to the friction roller.
Some friction rollers have a continuous elastomeric surface contacting the media along the roller length. Other friction rollers may include several elastomeric surfaces in parallel (e.g. "tyres") along the roller length. The friction roller preferably has a high friction surface. To achieve a high friction surface, it is desirable to provide a high coefficient of friction (COF) between the roller and the media sheet. Factors affecting the COF include the base material of the roller surface, adjunct materials added to or modifying the base material, the finish of the surface, and cleaning chemicals applied to the surface during its useful lifetime. A drawback with the friction roller is that over time the COF reduces due to paper fibres, dirt and other contaminants interacting with the roller surface.
FIG. 2 illustrates the sheet feed mechanism of FIG. 1 with the stack of media exhausted. The media stack may be replenished by manually sliding out the tray 10 in a direction B, adding a fresh stack of paper, and sliding the tray back to its operational position. The replenishing process has a drawback due to the floor of the tray dragging against the friction roller as the tray is slid out. This dragging action forces the friction roller in a clockwise direction opposite to the normal anti-clockwise pick direction of the roller, which in turn can result in permanent damage to the drive motor or the gear mechanism. Sometimes, the dragging action can be so strong as to strip the teeth from the gears of the gear mechanism.
In an attempt to solve the problem of the tray floor dragging against the roller, a known sheet feed mechanism includes a withdrawing mechanism which simultaneously lifts or withdraws the friction roller away from the sheet stack as the tray is withdrawn. However, the applicant has found in practice that users frequently remove the in-tray at excessive speeds, sometimes in order to replenish the media stack before the printer signals a "paper out" error. In these situations, the inertia of the withdrawing mechanism prevents the friction roller from being lifted quickly enough to avoid the dragging action of the tray floor. Accordingly, the motor and gear mechanism can still be damaged. Sometimes, the frictional force produced by the dragging action of the tray floor can actually hold the roller in place against the withdrawing action of the mechanism.